Conventional pumping units of the walking beam-type are designed to convert high speed, low torque rotary motion into low speed, high load reciprocating motion. These units typically include a walking beam, a prime mover, a gear reducer, crankarms attached to the gear reducer, pitman arms and an equalizer beam connecting the crank arms to the walking beam, a samson post which pivotally supports the walking beam, a horsehead attached to one end of the walking beam for supporting a polished rod load, crankarm counterweights, and various bearing assemblies. This type of pumping unit, which is generally referred to as a pump jack, has been used for many years to pump oil from vertical wells.
Recently, there has been some interest in bringing oil to the surface by means of off-vertical wells or slant hole wells. One advantage in using off-vertical wells to bring oil to the surface is that it requires the use of lens surface land. Less land is needed, since a large number of slant hole wells can be drilled adjacent to one another at a single central location (in a circular pattern, for example), in order to pump oil from a given formation, as opposed to a number of separate regularly spaced locations as is required for vertical wells. Thus, considerably less land is required for the pumping operation itself. In addition, the need for the use of land for access roads and the like is also correspondingly reduced. In some cases, as little as one tenth the land required for pumping purposes by vertical wells is needed in the case of pumping by off-vertical wells. Slant hole drilling is thus advantageous in urban areas or areas which are being farmed or otherwise utilized. It is also advantageous in swamp lands where it is difficult to construct mounting pads, or for drilling under rivers, lakes or offshore, where a vertical well would require the construction of an artificial island or the like. A slant hole well also enables the pumping of a formation which is located directly below a building or other obstruction. Slant hole drilling is particularly advantageous in drilling shallow wells which cannot be drilled by means of directional drilling (which involves drilling vertically for a considerable distance, then gradually deviating from the vertical).
Conventional pumping units of the walking-beam type are, however, designed to efficiently pump vertical wells only. Pumping a slant hole which deviates from the vertical by more than only a few degrees by means of an unmodified conventional pumping unit is unsatisfactory, for a number of reasons. The wire sling attached to the polished rod would not wind up and unwind cleanly along the horsehead arc, which would result in the imparting of a lateral deflection into the polished rod, which would damage the wellhead. The wire sling itself would tend to wear rapidly and break. Non-vertical forces greatly exceeding the design criteria of conventional pumping units would be exerted on the samson post. A clearance problem would also result, since the base of the pumping unit would have to be positioned too close to or on top of the well.
Some attempts have been made to pump off-vertical wells by means of pumping units of class III geometry having shorter than usual pitman arms. A class III unit is to be distinguished from a class I unit in that in the former unit, the pitman arms are connected to the walking beam between the samson post and the horsehead, whereas in the latter unit the samson post is connected to the walking beam between the pitman arms and the horsehead. Reference may be made to the "API Specification for Pumping Units" API STD 11E, Twelfth Edition, January 1982 regarding the distinction between a class I lever system and a class III lever system.
Shortening the pitman arms of the class III unit is directed at solving the above-noted problems of lateral deflection and rapid string water. However, reducing the length of the pitman arms to any extent increases the torque exerted on the gear reducer, thus reducing the efficiency of the pumping unit. As a result, gear reducers and motors of increased capacity are required. Operating costs are thus increased in areas in which electricity charges are based upon peak power consumption. Furthermore, shortening the pitman arms results in decreased wellhead clearance.